The IEEE 802.16e-2005 standard uses Time Division Multiplexing (TDM) to enable multiple users to be serviced with data transfer, coordinated by a Base Station. Synchronized time slots are allocated for data transmission from the BS to the Mobile Subscriber (MS), a direction of data flow known as a downlink, and for data transmission from the MS to the BS, known as an uplink. Uplink communications and downlink communications may use the same frequency band and are therefore differentiated by time slot. The 802.16e standard uses Orthogonal Frequency Division Multiplexing—Multiple Access (OFDMA) as the modulation method for transmitting data on the RF channel. These bursts of transmissions require the receiver to synchronize to the transmitter before data can be extracted from the signals. The Downlink sub-frame therefore starts with a Preamble and is followed by the data symbols.
FIG. 1 shows a preamble according to 802.16e standard where ‘N’ is the FFT size used in the downlink and ‘CP_LEN’ is the cyclic prefix associated with any OFDM symbol. The possible values of ‘N’ are 512 and 1024. Since N is not exactly divisible by three, the sample delay used is round(N/3) where round( ) is a round-off function to the nearest integer.
Each BS segment in a cell will transmit every 3rd sub-carrier in the preamble according to the segment allocated to the BS as shown in FIG. 2. It can be seen from FIG. 3 in combination with FIG. 2 that different adjacent segments use different sets of subcarriers, and that the configuration of FIG. 2 is a typical cell arrangement. One characteristic of the subcarriers as shown in FIG. 2 is that the Seg_0 subcarriers will accumulate 0 degrees of phase shift over an N/3 interval shown in FIG. 1, the Seg_1 subcarriers will accumulate 240 degrees of phase shift over an N/3 interval, and the Seg_2 subcarriers will accumulate 120 degrees of phase shift over an N/3 interval.
FIG. 4 shows the block diagram of a prior art wireless receiver. Antenna 405 is coupled to RF front end 404 which converts the signal to baseband and has adjustable gain controlled by AGC 402, which is operative over a part of the preamble. ADC 406 samples the baseband converted signal to an IQ baseband data stream referred to as RX_IQ. The main function of the synchronization block 408 is to detect the frame boundary using the pre-defined preamble of the RX_IQ stream. The remote transmitter boosts the preamble in power by 3 dB compared to the data symbols which follow the preamble. The frame preamble is a repetitive sequence specifically designed for robust detection and identification in a receiver. A typical implementation of preamble detection logic would use shifted auto-correlation between the repetitions in the preamble, whereby the preamble is correlated with time-shifted shifted version of itself with a shift of ‘N/3’ and the shifted correlation is accumulated over a window having an extent of N/3+N/8.
FIG. 5 shows the computation of shifted auto correlation for an 802.16e receiver, where the autocorrelator includes multiplier 510 which is multiplying first values from the first preamble part 504 with conjugated 511 second values from second preamble part 506, and the result of each multiply 510 is accumulated 512 over a window equal to N/8. The preamble can be detected by observing the output of the accumulator 512 in combination with a threshold to make the preamble detection decision. The shifted correlation is a complex value with a defined plateau. The width of the plateau would be CP_LEN (N/8) due to the accumulation of the correlation over a window of ‘N/8’.
The shifted correlation is computed as
      shifted_corr    ⁢          (      k      )        =            ∑              n        =        0                              CP          ⁢          _          ⁢          LEN                -        1              ⁢                  x        ⁡                  (                      k            -            n                    )                    ×                        x          *                ⁡                  (                      k            -            n            -            D                    )                    
Where CP_LEN is equal to ‘N/8’, and D is the delay representing the separation between repeating preamble symbols used for the shifted version of the preamble.
FIG. 6 shows the property of shifted auto-correlation such as the output 514 of accumulator 512. Magnitude 606 of the accumulator output is compared to a threshold level 604 for detection of preamble, and phase 602 shows a flat characteristic over the preamble correlation extent. As was described earlier, the phase plateau 603 for a Seg_0 segment will be 0 degrees, the phase plateau 603 for a Seg_1 segment will be 240 degrees, whereas the phase plateau 603 for a Seg_2 segment will be 120 degrees. The particular threshold value for preamble detection is based on the magnitude of the raw I and Q samples coming in, such that as the average signal energy increases, the threshold also increases so as to keep noise or other random patterns from triggering a false preamble-detect event.
FIG. 7 shows the block diagram for prior art preamble detection with multiple input receive streams, such as from multiple antennas as used in multiple input multiple output (MIMO) wireless systems. Preambles 716 and 718 represent the RX_IQ streams from a first and second ADC associated with a first and second antenna, and the two streams are separately conjugated (703, 707), multiplied (702, 706), and accumulated (704, 708). The first accumulator 704 and second accumulator 708 outputs are added 710 to form output 714, which is threshold compared, and the resultant value is used to determine the presence of a preamble. Multipliers 702 and 706 operate on complex values, and are used in combination with associated conjugators 703 and 707 to compute the dot-product of the complex preamble samples by multiplying them with delayed and conjugated versions. The conjugation operation 703 and 707 involves reversing the magnitude of the imaginary component. The resultant dot product which is accumulated 704 and 708 is also complex. The accumulation is carried out over a predetermined window (N/3), which windowing can be done many ways, including subtracting the dot-products of the N/3 delayed sample with a further N/3 delayed samples, thereby cancelling out old accumulated values and preserving the desired current window. Each accumulator is reset to zero when preamble detection is accomplished so as to enable it to start afresh for a new preamble. During preamble search of the noise values which precede the preamble, the accumulator adds the shifted autocorrelation values of noise in the inter-frame gap, and due to the non deterministic or random nature of noise the accumulator does not hold any significant value during that interval.
The technique of FIG. 7 works well in the absence of interferers since the shifted correlation values from the two antenna paths add up constructively to improve the correlation strength. FIG. 8 shows this in the form of vectors, where a signal 802 from the RX_IQ stream associated with antenna 1 is added to signal 804 associated with RX_IQ signal stream of antenna 2 antenna to generate combined signal stream 806.
FIG. 9 shows the phase relationship between the segments of a base station as was shown in FIG. 3. Each of Base stations C1 302, C2 304, C3 306 transmit simultaneously on different segments S0, S1, S2 and at different subcarrier combinations, as was shown in FIG. 2. Since each of the neighbor BS segments C1, C2, C3, etc transmit the preamble on every third sub carrier as shown in FIG. 2, the resulting phases of shifted correlation at the subscriber as shown in FIG. 9 will be 120 degrees apart. A parameter associated with a station is the “reuse number”, which indicates the number of frequencies deployed by the network. FIGS. 2 and 3, in a reuse-1 scenario, show that the cell is divided into spatial zones serviced by different subcarriers in the preamble. For reuse-1 interference the subscriber station might see the interference in all three segments from neighboring Base Stations.
FIG. 9 shows the signal received from each interferer for reuse-3 when the subscriber receives the signal from all Base Stations with equal power. As shown in the figure, if all the BS interferers are received with the same power 902, 904, 906, the resulting signal will be cancelled out and therefore the preamble cannot be detected. This is an exceptional scenario and is shown for example only. Naturally, the typical signal from each base station BS1, BS2, BS3 will have a different level due to different multipath channels as shown in FIG. 10, where base stations Bs1, Bs2, Bs3 sum to produce combined power 1116. The conventional combining of the Rx signal at the antennas will give better results in absence of BS Interferers but will prove ineffective in presence of Interferers. FIGS. 11A and 11B show signal streams combining at the first receive antenna Al and second receive antenna A2 in a typical multipath channel in presence of BS interferers. The individual antenna signals A1 1108 and A2 1116 of FIGS. 11A and 11B, respectively, combine as shown in FIG. 11C to produce a weaker signal 1120 than either contributor 1108 or 1106 individually. It is then clear that for certain signal conditions, the combined cross correlation of the individual RX_IQ streams combine in a destructive manner, thereby degrading the performance of the prior art preamble detection.